Flexible probe array transducers are commonly used for ultrasonic inspection of tubes, pipes or other curved surfaces. The flexible array often comprises a long ribbon-like acoustic module which is wrapped around the tube or pipe for detecting, for example, thickness variation in tubes, and/or cracks in pipes or welds.
The acoustic module usually comprises four layers: a composite piezoelectric array (hereinafter, the “composite”), a flexible printed circuit board (hereinafter, the “flexible circuit”), a backing material and a matching layer. The composite comprises multiple piezoelectric elements for transmitting and receiving ultrasonic energy. Each element of the composite is electrically contacted by a conductive layer, usually gold, and the flexible circuit is commonly used to conveniently route all electrical contacts to a suitable connector. The purpose of the backing material is to ensure that the acoustic module emits ultrasonic energy from one side only and to dampen the resonance of the elements. When the elements are well damped, shorter pulses of ultrasonic energy may be emitted. The matching layer is required on the emitting side of the acoustic module to ensure efficient transmission of ultrasonic energy from the composite into the material being inspected, and to prevent unwanted reflections so that the emitted signal can have shorter pulse width and larger bandwidth.
FIG. 1A is a schematic illustration of an acoustic module in existing practice, showing the four layers described in the previous paragraph. The acoustic module in existing practice has a number of problems and disadvantages. A first problem is related to the number of layers and the number of steps in the manufacturing process, both of which lead to high cost and long manufacturing lead times. In addition, the large number of layers impacts reliability of the final product because failure of glue joints between layers is a commonly known reliability problem.
A further problem with the acoustic module in existing practice is that the presence of the flexible circuit results in a lack of intimate contact between the composite and the backing material. This lack of intimate contact reduces the effectiveness of the backing material, resulting in increased ringing during ultrasonic emission and consequent loss of near-surface resolution.